knickerbocker glory
An ice cream dessert (a sundae) served in a tall glass, and consisting
of layers of chopped fruit and ice cream in layers, with
whipped cream on top. The dessert is eaten with a thin, long-handled
spoon. It's possible that 'Knickerbocker' refers to the shape of the
glass, which looks rather like the leg of a pair of knickerbockers.

They ate in the zoo restaurant, and when
Dudley had a tantrum because his
knickerbocker glory didn't have enough ice cream on top,
Uncle Vernon bought him
another one and Harry was allowed
to finish the first.
(PS2).

knickerbockers
Loose-fitting pants gathered at the knee or calf. Knickerbocker
was a nickname for the early Dutch settlers in New Amsterdam
(present day New York), who used to wear these fairly racy trousers.
Related: New York Knicks, knickers. See also
plus-fours.

"D'you want to be showing off your knickers in midair?
Because if you do, hang around...they're moving this way,
and it would give us all a laugh."
- Draco to
Hermione
(GF9)

Knight Bus
Play on words: "Night Bus," buses which run in London
during the night. See
full essay.
Also suggests the concept of "knight in shining armour" coming
to the rescue of a person in need, in this case a stranded witch or wizard.